Explore the Sunken Pirate Ship - not actually sunk, well not all of it. Find the prow of a ship on the sad by a rick with a hole in it. Next to it is a Ghost Pirate. Once the quest updates for finding the wreck, talk to him. He tells you that he is holding a voodoo doll that is keeping a Powerful Pyrkari subdued, and he will give it to you if you help him make a new body.
In addition to the Possessed Privateers near the chest with the Pirate Hat, there is a Thieving Pirate that will appear and attack, so make sure you pull any pirate you can see before you approach the chest!
The Skeleton has no such invisible protector.
When you give the items to the ghost he becomes a Zombified Pirate and attacks you (big surprise, huh? "Oh, help! I was not expecting that!"). When his new body has been defeated the ghost honors his deal and gives you the voodoo doll. Now you can use it to control the Powerful Pyrkari so you can take him to Farhall (you have 10 minutes).